WASHINGTON AND AMMAN, JORDAN
— From the beginning, US war commanders have said the fight for regime change in Iraq was going to be tough, risky, and potentially costly. "Ground truth," as Defense Secretary Donald Rumsfeld likes to put it, is proving them right in the form of coalition casualties and captured soldiers.

Some of the toughest Iraqi forces - Republican Guard units and the paramilitary "Fedayeen Saddam" - have come out to meet US and British forces, mingling among civilians, attacking supply-and-maintenance units, and forcing the kind of close-in urban fight the US and its allies had hoped to avoid.

"We are moving from shock and awe to attrition warfare," says Air Force Col. Sam Gardiner (ret.), referring to the kind of hard-slogging combat that tries to wear down an opponent. "If the pattern continues, this could be a tough fight."

Coalition commander Gen. Tommy Franks acknowledges "sporadic resistance in a number of places [from] isolated units and enclaves." But he insisted in a briefing at command headquarters in Qatar Monday that "our forces are moving in ways and places that we believe are just exactly right in accordance with a plan that is designed to be flexible."

And some analysts see the potential for longer-term gain in the fact that Saddam Hussein's more well-equipped, well-trained, and motivated units have challenged the US-led coalition far from Baghdad.

"If the Republican Guard is south of Baghdad and stays there, that's good news; we can fight them there with less risk to civilians," says defense analyst Michael O'Hanlon of the Brookings Institution. "It's more important than the bad news of a bit more southern resistance than we might have hoped for."

Other observers are more circumspect, some calling into question the basic US tactic of dashing 300 miles toward Baghdad with limited numbers of ground troops, trying to bypass cities along the way, and leaving noncombat support and supply units to follow along.

"There is obviously a problem at the moment, and if they don't stop to regroup shortly they will outrun their supply chain that is a prime target for Iraqi harassing parties" warns Charles Heyman, a former British Army officer who now works as an analyst for Janes Land Armies. You need a lot more force protection on the supply routes."

That approach is likely to shift as more combat troops join the invasion, and especially as tactical jets and helicopter gunships (one of which was shot down Monday) hit Iraqi military sites along the way. Meanwhile, the heavy bombing of command-and-control sites and other military targets in and around Baghdad is continuing.

But as military analyst Charles Pena of the Cato Institute in Washington points out, "At some point, we will run out of targets - probably very soon - and then we have to go into Baghdad."

Until then, the fight to get there continues, marked by continued resistance on the ground that appears to be greater than anticipated. And unless the US approach is as flexible as General Franks says it is, that could mean more losses and delays. "For one thing, this resistance bleeds off force - units have to be left behind to mop up and cities that don't declare themselves liberated have to be buffered," says retired Navy warship captain and Pentagon strategist Larry Seaquist.

Meanwhile, Mr. Hussein seems to have reappeared on Iraqi television, apparently fit and talking tough. Invading US and British forces "will lose more and more [soldiers] and they will not be able to escape lightly from their predicament," he said Monday. "These forces entered our lands and where they penetrated they became entangled, desert behind them," he added.

Such talk has the potential for not only boosting the morale of his forces, but more broadly influencing opinion in the region.

"It appears that Arab opinion is tipping faster and further into a hard, anti-invasion mood than expected," says Captain Seaquist. "US planners seemed to be expecting that their quick victory would carry the Iraqi public and temper the regional skepticism. Now, it appears, that broader anger, combined with the less than invincible impression from the fighting in the South, may help Saddam hold the Iraqi public onside - at least for awhile longer."

For now, US forces continue to push northward, knowing full well that other surprises may be in store for them.

"Resistance is predictably occurring at key points: Umm Qasr because it controls the Shat and is the main Iraqi port on the Gulf, and the bridges across the Euphrates," says US Army Col. Dan Smith (ret.). "Normally, bridges would be blown to stop invaders, which raises the possibility that the Iraqis may want to entice the US ground forces into the plains between the Tigris and Euphrates and then flood the area."

"I think US commanders may have thought they could roll close in to Baghdad given the Iraqi experience in 1991," he adds. "But as always, the enemy has a seat at the tactical table and can throw a wrench into the plan."

There were indications on Monday morning that Iraqi forces still posed a danger in areas through which US and British troops passed several days ago.

The US military canceled a planned trip for journalists to the Rumaylah oil field, saying it was unsafe. "There are still bad guys with guns. It's a war zone," Lt. Col. Jennifer Cassidy, a US military spokeswoman, told Reuters.

Group Capt. Al Lockwood, a British military spokesman, told reporters that a British convoy of marines had been ambushed in southern Iraq by "irregular forces providing difficulties for us with their guerrilla tactics." Yet he said that "we are maintaining our advance ... and will come back and look after them in our own time."